Process for electroplating metals

ABSTRACT

A process for electroplating metals in an electroplating cell in which the anode current efficiency of the cell is greater than the cathode current efficiency. Buildup of excess metal in the cell is counteracted by circulating the plating solution through an electrowinning cell and plating out the excess metal onto cathodes in the electrowinning cell. The amount of current flowing through the electrowinning cell is controlled so that the amount of the said current which results in the deposition of metal is at least substantially equal to the amount of current flowing through the electroplating cell which results in the evolution of hydrogen gas.

FIELD OF THE INVENTION

This invention relates generally to a process for electroplating metals.More particularly, the invention is concerned with an electroplatingprocess which can be performed in an electroplating cell having greateranode current efficiency greater than cathode current efficiency.

In the context of the present invention, the term "current efficiency"in relation to an electrode has its normal meaning in the art, namelythe ratio of the useful current transferred between the electrode andthe electrolyte to the current supplied to the electrode (usuallyexpressed as a percentage).

BACKGROUND OF THE INVENTION

Electroplating of metals is well known. A workpiece capable ofconducting an electric current is immersed in a bath containing asolution of metallic salts. A cathodic charge is imparted to theworkpiece by means of a source of direct electric current. A thin layerof metal contained in the solution is thus deposited on the surface ofthe workpiece. A counter-electrode or anode is required in the bath. Theanode may be soluble, in which case it is usually the same type of metalas is being deposited. Theoretically, metal is dissolved from the anodeat the same rate as it is deposited at the cathode. Alternately, theanode may be an insoluble type. In this case the anode material does notdissolve. The result of the anodic reaction is the generation of gas.Where metallic sulfate solutions are employed, the gas is predominantlyoxygen. Where appreciable concentrations of chloride are present,chlorine gas may also evolve.

U.S. Pat. No. 4,778,572 to Brown (the disclosure of which isincorporated herein by reference) indicates that, for plating processessuch as zinc and nickel using soluble anodes, the anodic currentefficiency is virtually 100%, meaning that essentially no gas is evolvedfrom the anode and all the electrical current passing through the anoderesults in the dissolution of metal from the anode. At the cathode, thesituation is somewhat different. Typically, a small, but significantpercentage of the current passing through the cathode results in theproduction of hydrogen gas instead of metal. Normally the cathodiccurrent efficiency varies between 90-98% in terms of metal deposition.

A difference between anode and cathode current efficiencies causes metalto dissolve from the anode faster than it plates onto the cathode. Thisleads to a build up in the concentration of dissolved metal in the bathsolution. Eventually the dissolved metal concentration rises to thepoint where it is deleterious to the plating process and solution mustbe decanted from the plating bath and replaced with water to reduce theconcentration to an acceptable level.

Coincidental with the rise in metal concentration in the electroplatingsolution is an increase in pH. The pH of the solution is a criticalfactor in maintaining satisfactory performance of the electroplatingprocess. Rising pH is normally counteracted through regular additions ofacid to maintain a constant pH in the bath.

Excess metal bearing solution decanted from an electroplating bath isenvironmentally hazardous and must be disposed of in an environmentallysafe manner. The conventional means of disposing of such metal bearingwastes is to raise the pH with an alkali such as sodium hydroxide orlime and precipitate the metals as metal hydroxide sludge. This processis very expensive to operate and results in the generation of largequantities of solid waste which must be disposed of. Disposal of solidwaste is becoming increasingly difficult and expensive. Moreover, theloss of metal value is significant.

A more attractive way to dispose of the excess solution is toelectrolytically treat the solution in an electrowinning cell to recoverthe metal. The electrowinning cell is equipped with insoluble anodes andcathodes whereupon metal is deposited. This procedure is commonlypractised in the electrolytic refining of copper. Metallic impuritiesbuild up in the copper containing electrolytes, necessitating bleedingoff a certain volume of solution. Most of the copper in this solution isplated out in electrowinning cells in a procedure known in the industryas "de-copperization". The remaining solution is then disposed of. Bythis procedure, the copper concentration of the solution is typicallyreduced from more than 50 g/L to less than 10 g/L.

A similar procedure is followed in electrolytic zinc plants, whereelectrolyte is sometimes purged from the system because of a build up ofmanganese impurities. The zinc concentration in the solution is reducedin so-called "stripping" cells prior to final neutralization anddisposal. Unfortunately, with this technique it is only possible toreduce the zinc concentration to a level of 10-20 g/L, so that aconsiderable quantity of zinc remains in solution.

There are a number of difficulties with this process:

As the metal is deposited at the cathode of the electrowinning cell,acid is generated at the anode and the pH of the solution drops. Whilecopper can be plated from highly acidic solutions to a relatively lowconcentration, other metals such as nickel, zinc and iron will notelectroplate efficiently under highly acidic conditions. To avoid thisproblem with these other metals, it is necessary to continuously adjustthe pH upwards through addition of an alkali such as sodium hydroxide.The cost of the alkali consumed in this process is appreciable and forthat reason this process is unattractive.

As the concentration of metal decreases in the electrowinning solution,the rate at which metal ions diffuse to the surface of cathodedecreases, as there is less driving force for this diffusion. For agiven current density, eventually a point is reached where the rate ofmetal deposition at the cathode is greater than the rate of metaldiffusion to the surface of the cathode and concentration of metal inthe solution immediately adjacent to the cathode will decrease wellbelow the concentration in the bulk solution. This phenomenon is knownas concentration polarization. Concentration polarization results in adeterioration of the quality of the deposit. Instead of a smoothadhering deposit, a powdery, poorly adhering deposit is produced. Insome cases, dendrites will grow from the cathode and short circuitagainst the anode. In addition the cathodic current efficiency will bereduced as hydrogen gas begins evolving instead of metal deposition. Toavoid concentration polarization it is necessary to reduce the currentdensity in proportion to the reduction in the metal concentration. Thisincreases the size of the electrowinning cell required to remove a givenquantity of metal. It is not in fact practical to reduce the metalconcentration to the level where the solution would be acceptable fordischarge to the environment and supplemental conventional precipitationtreatment of the solution after electrowinning would be necessary. Forthis reason only a portion of the metal can be recovered.

If the plating solution contains significant quantities of chlorideions, chlorine gas will evolve at the insoluble anode in addition to, orinstead of oxygen. Chloride ions are very corrosive to most commoninsoluble anode materials, such as lead, and chlorine gas is highlytoxic. Provision must be made in the design of the electrowinning cellto handle the chlorine gas generated.

Thus it can be readily seen that use of an electrowinning cell in theusual manner is not a viable means of dealing with the problem of metalconcentration buildup in the electroplating bath.

The Brown '572 patent (supra) teaches one way to solve the problem ofincreasing metal concentration in the electroplating bath. By thismethod a small percentage of the soluble anodes in the electroplatingbath are replaced with insoluble anodes. The quantity of insoluble anodematerial is selected so that the overall anode efficiency in terms ofmetal dissolution is equal to the cathode metal deposition efficiency.Consequently, metal dissolves from the anodes at the same rate that itdeposits on the cathodes and no buildup occurs.

In some soluble anode plating processes it is not feasible to install asmall percentage of insoluble anode material as taught in theaformentioned patent. For example, in the so-called"electro-galvanizing" process, strip steel is continuously plated withmetals such as zinc, zinc-iron, or zinc-nickel alloy. The currentdensities are extremely high and the geometry of the anode is criticalto achieving an even current distribution at low operating voltage.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an improvedelectroplating process which is capable of accommodating differencesbetween anode current efficiency and cathode current efficiency in thean electroplating cell without the use of supplementary insoluble anodematerial in the electroplating cell.

Accordingly, the present invention provides a process for electroplatingmetals in an electroplating cell comprising a bath containing a platingsolution of a metallic salt, a cathode comprising a workpiece to beplated, and a soluble anode, and in which the anode current efficiencyof the cell is greater than the cathode current efficiency. The processincludes the steps of providing an electrowinning cell which includes atleast one insoluble anode, at least one insoluble cathode and a bathwhich communicates with the bath of the electroplating cell forpermitting circulation of the plating solution between the cells. Asource of direct electric current is connected to the anode and cathodeof the electroplating cell so a to cause electroplating of metal ontothe workpiece. A second source of direct current is connected across theanode and cathode of the electrowinning cell so as to cause depositionof metal from the plating solution onto the cathode. The platingsolution is circulated between the cells and the amount of currentflowing through the electrowinning cell which results in the depositionof metal is controlled to be at least substantially equal to the amountof current flowing through the electroplating cell which results in theevolution of hydrogen gas.

By controlling the current flowing in the electrowinning cell in thisway, the rate of metal deposition in the electrowinning cell issubstantially the same as the rate of dissolved metal buildup in theelectroplating cell solution. Accordingly, the buildup in theelectroplating cell will be counteracted by a depletion of metal in theelectrowinning cell. Circulation of the plating solution between the twocells will substantially avoid excess metal buildup in theelectroplating cell.

The metal that has been deposited on the cathode in the electrowinningcell can be recovered and re-used as anode material in theelectroplating cell, or sold to recoup its value.

It is not possible to utilize the process of the Brown paten (supra)when electroplating more noble metals. Because the electrical potentialfor anodic dissolution of these metals is so low, insoluble anodes willnot carry any appreciable current when connected to a common electricalbus with the soluble anodes. A case in point is copper electroplatingfrom acid sulfate electrolytes. The present invention provides a meansof dealing with the differential in anode and cathode efficiencies insuch electroplating processes, since the current in the electrowinningcell can be set to the desired level by adjustment of its own rectifier.The current is therefore totally independent of the electrical potentialexisting in the electroplating cell.

For the same reason a further advantage is found with the presentinvention. Even with metals such as nickel, with the previous processthe current density carried by the insoluble anodes will under normalcircumstances be no more than the current density carried by the solubleanodes, and quite frequently will be appreciably lower. It is possibleto operate the electrowinning cell according to the present invention atanode current densities well in excess of the anode current densities inthe electroplating cell. As a result, the area of insoluble anodesrequired can be reduced to an amount significantly less than thatrequired if the previous process were utilized. Other equipment can bereduced in proportion to the reduction in anode area. This will reducethe overall cost of the system.

BRIEF DESCRIPTION OF THE DRAWINGS

In order that the invention may be more clearly understood, referencewill now be made to the accompanying drawings which illustrateparticular preferred embodiments of the invention by way of example, andin which:

FIG. 1 is a diagrammatic illustration of an apparatus for performing theprocess of the present invention;

FIG. 2 is a diagrammatic illustration of a modified form ofelectrowinning cell for use in the process; and,

FIG. 3 illustrates a modified form of the apparatus shown in FIG. 1which permits recycling of "dragout" losses.

DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to the drawings, FIG. 1 illustrates a process for continuouslyplating strip steel with a metal such as zinc, zinc-iron, or zinc-nickelalloy. The process is carried out in a electroplating cell which isgenerally indicated by reference numeral 20 and which includes a bath 22containing a plating solution 24 of a metallic salt (e.g. a zinc salt).Steel strip to be plated is shown at 26 and is continuously conveyedthrough solution 24 around rollers generally indicated at 28, 30, 32 and34. Bath 22 also includes a soluble metal anode 36 (e.g. of zinc).

A cathodic charge is imparted to the travelling steel strip 36 byconnecting a source of direct electric current between strip roller 28and anode 36, as indicated at 38. As is well known per se, metal fromsolution 24 is deposited on strip 26 as it travels through the solution.At the same time, metal is dissolved from anode 36 into solution 24. Asdiscussed previously, a difference between anode and cathode currentefficiency causes metal to dissolve from the anode faster than it platesonto the cathodic steel strip. This leads to a buildup in theconcentration of dissolved metal in solution 24.

In accordance with the present invention, an electrowinning cell 40 isprovided and includes a bath 42 which is connected to bath 22 of theelectroplating cell by pipes indicated at 44 and 46 so that platingsolution from bath 22 can be circulated through the bath 42 of theelectrowinning cell 40. Pumps P are provided in both pipes forcirculating the plating solution.

Cell 40 includes a series of insoluble anodes 48 connected to a commonbus bar 50 and a series of intervening cathodes 52 connected to a commonbus bar 54. A source of direct electric current separate from source 38is connected across the anode and cathode bus bars 50, 54 as indicatedat 56, so as to cause deposition of metal from the plating solution inbath 42 onto the cathodes 52. Plating solution is circulated between thecells while the amount of current flowing through the electrowinningcell is controlled so that the amount of the current which results inthe deposition of metal is at least substantially equal to the amount ofcurrent flowing through the electroplating cell which results in theevolution of hydrogen gas. Consequently, the rate of metal deposition inthe electrowinning cell 40 will be the same as the rate of dissolvedmetal buildup in the electroplating cell 20. In other words, metalbuildup in the electroplating cell will be counteracted by a depletionof metal in the electrowinning cell.

The cathodes 52 of the electrowinning cell may be sheets of the metalbeing electroplated (e.g. zinc) or blank sheets of another metal such asstainless steel, titanium or aluminum, from which metal deposited can beeasily stripped. As such, the metal can be recovered and re-used asanode material in the electroplating process, or sold to recoup itsvalue.

The anodes 48 may be graphite, precious metal coated valve metal,precious metal coated ceramic materials, lead or a lead alloy.

If the cathode current efficiency in the electrowinning cell (E_(w)) isless than 100% the electrowinning current will have to be increased tocompensate. The electrowinning current required in the electrowinningcell (I_(w)) can be calculated from the following equation. ##EQU1##where,

E_(p) =cathode efficiency in electroplating cell (%)

E_(w) =cathode efficiency in electrowinning cell (%)

I_(p) =current in electroplating cell (amp)

I_(w) =current in electrowinning cell (amp)

If the electrowinning cell current is too high, the metal concentrationin the solution will decrease and the pH will drop. If theelectrowinning cell current is too low, the metal concentration willstill tend to rise and the pH will tend to rise, although not as quicklyas would occur if no electrowinning cell were employed.

Under practical circumstances it may be difficult to adjust theelectrowinning cell current to exactly the optimum level. The idealeffect can be obtained by adjusting the current up or down from time totime to maintain metal concentration and pH within acceptable limits. Inthis case, the average current through the electrowinning cell will beessentially the same as the amount of current flowing through theelectroplating cell which results in the evolution of hydrogen at thecathode in the electroplating cell.

For a sulphate type plating solution, electrolysis of water at theinsoluble anodes in the electrowinning cell will result in thegeneration of oxygen and sulfuric acid. This acid will neutralize thehydroxyl ions that are generated in the electroplating cell coincidentalwith the evolution of hydrogen at the cathode. A side benefit of thisinvention then is that it is no longer necessary to make regularadditions of acid to the plating bath to counteract the tendency for theelectroplating solution pH to rise.

The flow rate of solution circulated between the electroplating bath andthe electrowinning cell is important. Preferably, the liquid iscontinuously circulated to avoid an increase in the metal concentrationand pH in the electroplating cell. If the flow rate is too low, the pHin the electroplating cell may become too high and the pH in theelectrowinning cell may become too low.

The minimum recirculation flow rate required is dependent on thefollowing factors:

the current required in the electrowinning cell

the maximum pH or minimum hydrogen ion concentration allowable in theelectroplating bath, [H]_(p)

the minimum pH or maximum hydrogen ion concentration allowable in theelectrowinning bath ([H]_(w)) that will allow deposition of metal atgood cathode efficiency and good deposit quality. This minimum pH willdepend on the composition of the solution, particularly, the standardelectrode potential of the metal being deposited and its concentrationas well as other factors such as the current density, temperature andthe presence of chemical additives that will effect the morphology ofthe deposit.

the percentage cathode current efficiency in the electrowinning cell,E_(w).

The minimum recirculation flow rate can be calculated from the followingequation: ##EQU2## where,

F=Faraday's constant=96,500

[H]_(w) =maximum hydrogen concentration in electrowinning cell (N)

[H]_(p) =maximum hydrogen concentration in electroplating cell (N)

For example, in the case of zinc plating, the electroplating bath willoperate at a pH of approximately 4 which corresponds to a hydrogen ionconcentration of about [H]=0.0001 N. The minimum pH for efficient zincelectrowinning may be about 1.5, which corresponds to a hydrogen ionconcentration of about [H]=0.0316 N. The cathode current efficiency inthe electroplating cell will be about 95%.

For an electroplating bath with a total current of 100,000 amperes theminimum flow rate required would then be calculated from the aboveequation as 6233 litres per hour.

The actual minimum pH for the electrowinning cell must be determined foreach individual case as the conditions can vary quite widely. Accordingto equation (2), the flow rate is strongly dependent on the acidconcentration in the electrowinning cell. Normally the acidconcentration in the electrowinning cell will be much higher than thatin the electroplating cell, so that the minimum flow will be inverselyproportional to the acid concentration in the electrowinning cell. Thismeans that under normal conditions, the flow rate must be increased by afactor of 10 for each pH unit increase required in the electrowinningcell. In those cases where the electrowinning pH must be relativelyhigh, the flow rate must be quite high.

Theoretically, there is no maximum allowable recirculation flow rate,although there are obviously practical limits to how much liquid can bereasonably pumped.

The current density employed in the electrowinning cell should bemaximized while achieving as high a current efficiency as possible andproducing an acceptable deposit. Although a smooth, adhering depositwould normally be considered ideal, under some circumstances, a powdery,loosely adhering deposit may be preferable. Electrowinning cells areavailable which are designed to handle production of such powders.

If chloride ion concentration in the plating solution is sufficientlyhigh, chlorine gas will be generated at the anodes in the electrowinningcells in addition to, or instead of oxygen. In this case, theelectrowinning cell design should make provision for the chlorine as thechloride ion is very corrosive to many anode materials and chlorine gasis extremely toxic. Anodes fabricated from graphite, precious metalcoated valve metal such as iridium oxide coated titanium, or preciousmetal coated ceramic materials, which are resistant to chloride ions andchlorine can be employed.

Provision should also be made for collection of the chlorine gas evolvedfrom the anode in the electrowinning cell in a safe manner. This can beaccomplished by installing a hood over the electrowinning cell or overeach individual anode.

Alternatively, the anodes in the electrowinning cell can be enclosed inindividual anode chambers or compartments formed by cation exchangemembranes, as shown in FIG. 2. This view shows a modified form of theelectrowinning cell of FIG. 1. In FIG. 2, primed reference numerals havebeen used to denote parts that correspond with parts shown in FIG. 1. Ain that embodiment, the electrowinning cell 40' includes a series ofanodes 48' and a series of intervening cathodes 52'. The anodes andcathodes are connected to respective bus bars but the bus bars have notbeen shown in FIG. 2.

Each of the anodes 48' is provided with an enclosure 58 formed by acation exchange membrane. A sulfuric acid anolyte 60 surrounds the anodewithin enclosure 58. FIG. 2 also diagrammatically illustrates at 62 anoxygen or chlorine gas collection system that communicates with theinterior of each of the enclosures 5 as indicated at 64. The enclosures58 can be bags of membrane material with or without supporting frames.

The cation exchange membrane from which the enclosures 58 are made mustbe resistant to chlorine. An example of a suitable material is NAFION(TM), a product of E. I. Du Pont Co., Inc. This is a perfluorosulfonicacid type membrane. Dilute sulfuric acid, at a concentration of 0.1 N to5, can be employed as an anolyte in the anode compartment. Use of amembrane anode compartment to separate the anolyte from the catholyte orplating solution in this manner will substantially reduce the quantityof chlorine gas produced at the anode. Although in principle thereshould be no chlorine gas produced if this technique is followed, thediffusion of small quantities of chloride ions through the cationmembrane is unavoidable and in practice a limited amount of chlorine gaswill be produced. This will necessitate provision for collection of thechlorine (system 62).

An additional benefit of using the ion exchange membrane for zinc/ironor other ferro-alloy plating solutions is that the iron contained in theplating solution does not contact the electrowinning anodes directly,thus preventing oxidation of iron from the divalent ferrous valencestate to the trivalent ferric state. Ferric iron is undesirable in theelectroplating process since cathodic reduction of iron from thetrivalent state back to the divalent ferrous state consumes electricalenergy which could otherwise be used for plating of metals.

Prevention of metal buildup in the electroplating solution also makes itpossible to recycle so-called "dragout" losses.

Thus, as the steel strip leaves the electroplating bath a certain amountof plating solution adheres to the surface and must be subsequentlyrinsed off. This phenomenon is commonly referred to as "dragout". Themetal salt content of the rinsewater represents a pollution hazard aswell as a significant economic loss. It would be highly advantageous torecycle this rinsewater back to the plating bath. A number of processesincluding ion exchange, electro-dialysis and evaporation are availableto facilitate recovery of the rinsewaters. Rinsewater recycle isnormally not feasible, however, because recycle of metal back to theelectroplating bath would merely serve to accelerate the rate of buildupof dissolved metal concentration. However, dragout recycle can beaccommodated in the process of the invention, as shown in FIG. 3.

In FIG. 3, double primed reference numerals have been used to denotethat correspond with parts shown in FIG. 1. The electroplating cell andthe electrowinning cell are essentially the same as shown in FIG. 1 andare denoted respectively 20' and 40'. The steel strip 26', on leavingthe electroplating cell, is lead through a rinse tank 66 for removal of"dragout" from the strip. Water is added to tank 66 as indicated at 68and metal bearing rinsewater leaves the tank at 70 and is delivered to arecovery system 72. System 72 may comprise a conventional ion exchange,electro-dialysis or evaporation apparatus. Reclaimed rinsewater isreturned from system 72 to the electroplating tank 22' through a line 74which includes a pump P₁.

It will of course be appreciated that the preceding description relatesto particular preferred embodiments of the invention and that manymodifications are possible. For example, reference has been made to aprocess for electroplating certain specific metals and alloys but it isto be understood that the invention is not limited to these particularmaterials. Examples of other specific materials that may be plated arezinc, zinc/iron, nickel/iron, zinc/nickel, tin/nickel, cadmium. Theinvention has particular advantages for plating metals that can exist intwo valence states, such as iron, tin and copper. It is necessary to usea cation exchange membrane (FIG. 2) with such metals to avoid anodicoxidation to the higher valence state.

Referring specifically to the embodiment of FIG. 2, reference was madeto the use of dilute sulphuric acid as an anolyte. It should be notedthat any dilute strong acid can be used, e.g. sulphonic acid. Thespecific concentration of 0.1-5 N given in the disclosure is a preferredconcentration range and is not essential.

I claim:
 1. A process for electroplating metals in an electroplating cell which comprises a bath containing a plating solution of a metallic salt, a cathode comprising a workpiece to be plated, and a soluble anode, and in which the anode current efficiency of the cell is greater than the cathode current efficiency;said process comprising the steps of: providing an electrowinning cell which includes at least one insoluble anode, at least one insoluble cathode and a bath which communicates with the bath of said electroplating cell for permitting circulation of said plating solution between said cells; connecting a source of direct electric current across the anode and cathode of said electroplating cell so as to cause electroplating of metal onto said workpiece; circulating said plating solution between said cells; connecting a source of direct current across said anode and cathode of the electrowinning cell so as to cause deposition of metal from said plating solution onto said cathode; and, controlling the amount of current flowing through the electrowinning cell which results in the deposition of metal to be at least substantially equal to the amount of current flowing through the electroplating cell which results in the evolution of hydrogen gas.
 2. A process as claimed in claim 1, wherein said plating solution is continuously circulated between the electroplating cell and the electrowinning cell.
 3. A process as claimed in claim 1, wherein the electrowinning cell includes a plurality of anodes, each of which is isolated from the plating solution by a cation exchange membrane which defines an anode compartment containing an anolyte comprising a dilute strong acid.
 4. A process as claimed in claim 3, wherein the metal being plated is a metal that can exist in two valence states, wherein the lower valence state is preferred.
 5. A process as claimed in claim 3, wherein said dilute strong acid is a sulfuric acid.
 6. A process as claimed in claim 5, wherein the sulfuric acid anolyte has a concentration of 0.1-5 N.
 7. A process as claimed in claim 3, wherein said cation exchange membrane is a cation permeable, perfluorosulfonic acid type membrane.
 8. A process as claimed in claim 1, wherein the electrowinning cell includes a plurality of anodes which are made from materials selected from the group consisting of graphite, precious metal coated valve metal, precious metal coated ceramic material, lead and lead alloys.
 9. A process as claimed in claim 1, comprising the further steps of: removing the cathode from the electroplating cell after electroplating has been completed; rinsing the cathode with water to remove dragout of said plating solution; treating said rinsewater to remove metal and produce reclaimed rinsewater; and recycling said reclaimed rinsewater to said electroplating cell.
 10. An apparatus for electroplating metals comprising:an electroplating cell including a bath for containing a plating solution of a metallic salt, a cathode comprising a workpiece to be plated, and a soluble anode, the anode current efficiency of said cell being greater than the cathode current efficiency; an electrowinning cell which includes a bath .[.and.]..Iadd., .Iaddend.at least one insoluble anode; .Iadd.and at least one insoluble cathode;.Iaddend. means for coupling said electroplating cell bath with said electrowinning cell bath for permitting circulation of said plating solution between said cells; a source of direct electric current for connection across the anode and cathode of the electroplating cell for electroplating a metal onto said workpiece; .[.and,.]. a source of direct current for connection across said anode and cathode of the electrowinning cell so as to cause deposition of metal from said plating solution onto said cathode.[...].ep .Iadd.; and, .Iaddend. .Iadd.means for controlling the amount of current flowing through the electrowinning cell which results in deposition of metal to be at least substantially equal to the amount of current flowing through the electroplating cell which results in the evolution of hydrogen gas..Iaddend.
 11. An apparatus as claimed in claim 10, further comprising means for continuously circulating said solution between the electroplating cell and the electrowinning cell.
 12. An apparatus as claimed in claim 10, wherein the electrowinning cell includes a plurality of anodes, each of which is isolated from the plating solution in use by a cation exchange membrane which defines an anode compartment for containing an anolyte comprising a dilute strong acid.
 13. An apparatus as claimed in claim 12, wherein said cation exchange membrane is a cation permeable, perfluorosulfonic acid type membrane.
 14. An apparatus as claimed in claim 10, wherein the electrowinning cell includes a plurality of anodes which are made from materials selected from the group consisting of graphite, precious metal coated valve metal, precious metal coated ceramic material, lead and lead alloys.
 15. An apparatus as claimed in claim 10, further comprising means for rinsing with water said cathode after removal thereof from said electroplating cell to remove dragout of plating solution from said cathode; means for treating said water after rinsing of said cathode, to remove metal therefrom and produce reclaimed rinsewater; and means for recycling said reclaimed rinsewater to said electroplating cell. 